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During the handling of a key event, an IME may call SetComposition. IMF will
delay the processing of SetComposition until the key event is handled.

However, when a call to SetComposition("") is delayed, it is run as
ClearComposition() when the key event is handled. This can cause issues because
SetComposition("") has different behaviour from ClearComposition().
Specifically, SetComposition("") deletes any selected text, whereas
ClearComposition doesn't.

The root cause is that the code uses empty composition text to indicate that
there's a pending ClearComposition. So it's impossible to distinguish between a
pending SetComposition("") from a ClearComposition().

Fix this by putting the pending composition into an Optional, so that nullopt
means ClearComposition and anything else is SetComposition.

Bug: b/172284307
Change-Id: Iec39e8124eab6bd897d543f66f1a4bdb71b3fe78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2520556Reviewed-by: Shu Chen <shuchen@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825253}